1. Field of the Invention
The present invention generally relates to apparatus for making a hologram scale and, more particularly, is directed to an apparatus for making a hologram scale, an apparatus for making an assembled hologram scale, a hologram, an assembled hologram scale and a moving member having a hologram scale for use with a numerical control (NC) machine tool, a precision length measuring machine or a high accuracy displacement measuring apparatus.
2. Description of the Prior Art
Recently, a laser scale formed by the combination of a semiconductor laser and a hologram scale has become popular as a displacement measuring apparatus of high precision because it provides high resolution and excellent stability.
A conventional hologram scale is constructed as, for example, shown in FIG. 1.
Referring to FIG. 1, there is provided a laser light source 1 which emits a laser light. The laser light emitted from the laser light source 1 is reflected by a mirror 2 and then split by a beam splitter 3. The thus split laser beams are respectively reflected by mirrors 4, 5 and introduced to beam expanders 6, 7 provided as large aperture magnifying optical systems, in which they are expanded to provide an object wave L.sub.0 and a reference wave L.sub.R as plane wave. The object wave L.sub.0 and reference wave L.sub.R are superimposed to produce an interference fringe 8. This interference fringe 8 is exposed on a recording material 9 as a hologram scale, thereby forming the hologram scale.
In the conventional apparatus for producing the hologram scale, however, the optical path of the laser light for making the interference fringe 8 is relatively long. There is then the problem that linearity and average grating pitch of the thus produced hologram scale fluctuate because of the change of ambient temperature or the change of environmental conditions, such as the flow of air or the like.
Particularly, when the recording material 9 shown in FIG. 1 is moved in the direction shown by an arrow A in FIG. 1 in order to produce a long hologram scale or the like, a lot of time is needed to produce the long hologram, which unavoidably produces nonlinear.
Further, since the dimension of the optical system is relatively large, there is then the problem that linearity fluctuates even because of a very small vibration of the optical system. Accordingly, the hologram scale recording apparatus must be designed so as to have an expensive earthquake-resistant structure. However, even when the hologram scale recording apparatus is arranged as the earthquake-resistant type, it is very difficult to construct the hologram scale of high accuracy.
Since a magnifying optical system having a large aperture is needed, the interference angle cannot be made constant without difficulty.
When the exposure and recording are carried out again by moving the recording material 9 shown in FIG. 1 in the direction shown by the arrow A in FIG. 1 after the interference fringe 8 is exposed once on the recording material 9 in order to construct the long hologram scale, that is, when a so-called continuous exposure and/or recording is carried out, it takes a lot of time to construct the hologram scale. Besides, since the interference fringe 8 is not stable, then it becomes very difficult to match the phases of the two interference fringes formed on the recording material 9 with high accuracy. As a consequence, there is then the problem that the long continuous hologram scale of high precision cannot be constructed without great difficulty.
As earlier noted, since the dimension of the optical system of the apparatus for making the long continuous hologram scale is increased relatively, the linearity of the hologram scale fluctuates even because of a very small vibration of the optical system. Accordingly, the conventional continuous hologram scale must be designed so as to have an expensive earthquake-resistant structure. Even when such expensive earthquake-resistant structure is employed, it is impossible to construct the long continuous hologram scale without difficulty.
Furthermore, it is proposed that the continuous recording is carried out while the phase is being observed by an interferometer unitarily formed with the recording material 9 which constructs the continuous hologram scale forming apparatus. In this case, however, the vibration on the recording material 9 and the vibration of the wave surfaces caused by the flow of air at the portion where the object wave L.sub.0 and the reference wave L.sub.R overlap each other are not the same in cycle and in phase, so that the continuous recording cannot be made properly.